15.11. Can I filter/restrict/block certain websites with m0n0wall?

There are no filtering capabilities built into m0n0wall based on web
site content, keywords, etc., nor any supported add-ons with such
functionality.

Blocking by IP
Address/Subnet

You can block specific sites by putting in firewall rules to deny
access to the undesired server's IP address. If you take this path, it is
recommended you use "reject" rather than "block" in the firewall rules so
inaccessible sites time out immediately.

Blocking by DNS Override

If you use your m0n0wall as your only DNS server, you can also block
specific sites by putting in DNS override for the undesired site to point
to an internal or invalid IP address. To block www.example.com, put in a
DNS override pointing it to 1.2.3.4 or some other invalid IP address, or
an address of a LAN web server. If you use an invalid IP address, you
should put in a firewall rule to reject packets to this address so the
requests time out immediately.

Note this is easy to get around by either using a different DNS
server or editing the hosts file on the local machine, though this is
beyond the capabilities and knowledge of most any user.

Using a Proxy Server

The ideal solution would be to use a proxy server on your LAN, and
block outgoing traffic from your LAN hosts other than the proxy
server.